Recently developed flat panel detectors have been proven to have a much better image quality than conventional electronic portal imaging devices (EPIDs) [see, for example, G. Pang, D. L. Lee, and J. A. Rowlands, “Investigation of direct conversion flat panel imager for portal imaging”, Med. Phys., 28, 2121–2128 (2001)]. They are, however, not yet ideal systems for portal imaging application (using megavoltage x-rays) due to the low x-ray absorption, i.e., low quantum efficiency (QE), which is typically on the order of 2–4% as compared to the theoretical limit of 100%. The low QE is due to the fact that the total effective thickness (also referred to as the x-ray path length) of these detectors is only ˜2 mm while the first half value layer (HVL) for, e.g., 6 MV x-ray beam is ˜13 mm of lead. A significant increase of QE is desirable for applications such as a megavoltage cone-beam computed tomography (MVCT) and megavoltage fluoroscopy. However, the spatial resolution of an imaging system usually decreases significantly with the increase of QE. The key to the success in the design of a high QE detector is, therefore, to maintain the spatial resolution. Recently, we demonstrated theoretically that it is possible to design a portal imaging detector with both high QE and high resolution [see G. Pang and J. A. Rowlands, “Development of high quantum efficiency flat panel detector: Intrinsic spatial resolution”, Med. Phys. (In press, 2002)]. However, how to design a practical detector of this kind remained open.
Therefore, it would be very advantageous to provide an x-ray detector with both the quantum efficiency and spatial resolution that can be used for portal imaging applications including megavoltage cone beam computed tomography (MVCT).